1. Field of the Invention
The present invention relates to a method of controlling a PI (P: proportional, I: integral) or PID (D: derivative) action used in various process instrument control systems and an apparatus thereof and, more particularly, to a method of controlling a process and an apparatus thereof including a technical means for correcting the gain of a control loop in accordance with a change in gain of a control target and a change in disturbance.
2. Description of the Related Art
The basic equation of PI control according to process control is represented by equation (1): EQU MV=K.sub.c =(e+1/T.sub.I .intg.edt)+MV.sub.0 ( 1)
where MV: control signal, MV.sub.0 : initial operation signal, e: deviation, K.sub.c : proportional gain of controller, and T.sub.I : integral time.
When equation (1) is represented by a transfer function, equation (2) is obtained: EQU C(s)=MV(s)/E+K.sub.c {1+(T.sub.I .multidot.s)} (2)
where C(s): transfer function of PI control, s: Laplace operator. When the transfer function in equation (2) is represented by a velocity-type algorithm, equation (3) is obtained: EQU MV.sub.n =MV.sub.n-1 +.DELTA.MV.sub.n ( 3) EQU .DELTA.MV.sub.n =K.sub.c {(e.sub.n -e.sub.n-1)+(.DELTA.t/T.sub.I)e.sub.n }(4)
where .DELTA.t: control period, e.sub.n : current deviation signal, e.sub.n-1 : previous deviation signal, MV.sub.n : current operation signal, MV.sub.n-1 : previous operation signal, and .DELTA.MV.sub.n : change in current operation signal.
FIG. 1 is a view showing the arrangement of a conventional PID control apparatus.
This PID control apparatus inputs a set point signal SV.sub.n and a controlled variable signal PV.sub.n detected from the output side of a controlled system 1a of a process 1 through a controlled variable detecting mean 2 to a deviation calculating mean 3. The deviation calculating mean 3 calculates a deviation signal: EQU e.sub.n =SV.sub.n -PV.sub.n
to output the deviation signal to a velocity-type PI adjustment calculating mean 4. The velocity-type PI adjustment calculating mean 4 performs velocity-type PI adjustment calculation represented by equation (4) to calculate a change .DELTA.MV.sub.n in current operation signal to output the change .DELTA.MV.sub.n to a velocity-type to positional signal converting mean 5. The velocity-type to positional signal converting mean 5 extracts a positional manipulated signal MV.sub.n to output it to the process 1, and controls the controlled system 1a of the process 1 such that the deviation signal e.sub.n =SV.sub.n -PV.sub.n =0 is established by the positional manipulated signal MV.sub.n, i.e., set point signal SV.sub.n =controlled variable signal PV.sub.n is established.
Various disturbance signals D are added to the output side of the controlled system 1a through a disturbance transfer function 1b.
In the above control apparatus, during predetermined adjustment, e.g., in a temperature control apparatus in summer or winter, a PI parameter of the velocity-type PI adjustment calculating mean 4 is adjusted such that optimal control is performed depending on seasons, and the gain of a control loop is set to be an optimum value during the adjustment.
In the above control apparatus, when the controlled system 1a is kept in a predetermined adjusted characteristics, no problem is posed. However, in an actual plant, the gain of the controlled system 1a or the disturbance transfer function 1b is rarely constant, and a controlled system gain considerably changes depending on a change in operation point, a change in quality of a material or catalyst, a change in environmental condition, a change in load, and the like, thereby adversely affecting controllability.
In particular, in order to cope with an advanced, flexible plant operation, the above problems must be solved.